mysqljoin语句用法（MySQL的join buffer原理）

Table name      Type
t1              range
t2              ref
t3              ALL
The join is then done as follows:
 
- While rows in t1 matching range
 - Read through all rows in t2 according to reference key
  - Store used fields from t1, t2 in cache
  - If cache is full
    - Read through all rows in t3
      - Compare t3 row against all t1, t2 combinations in cache
        - If row satisfies join condition, send it to client
    - Empty cache
 
- Read through all rows in t3
 - Compare t3 row against all stored t1, t2 combinations in cache
   - If row satisfies join condition, send it to client

	

		

			
	

static int
join_init_cache(THD *thd,JOIN_TAB *tables,uint table_count)
{
……
  for (i=0 ; i < table_count ; i++)
  {
    bool have_bit_fields= FALSE;
    uint null_fields=0,used_fields;
    Field **f_ptr,*field;
    MY_BITMAP *read_set= tables[i].table->read_set;
    for (f_ptr=tables[i].table->field,used_fields=tables[i].used_fields ;
 used_fields ;
 f_ptr++)
    {
      field= *f_ptr;
      if (bitmap_is_set(read_set, field->field_index))
      {
used_fields--;
length+=field->fill_cache_field(copy);
……
      }
  }
 
  cache->length=length+blobs*sizeof(char*);
  cache->blobs=blobs;
  *blob_ptr=0; /* End sequentel */
  size=max(thd->variables.join_buff_size, cache->length);
  if (!(cache->buff=(uchar*) my_malloc(size,MYF(0))))
    DBUG_RETURN(1); /* Don't use cache */ /* purecov: inspected */
  cache->end=cache->buff+size;
  reset_cache_write(cache);
  DBUG_RETURN(0);
}

	

		

			
	

tsecer_select()
{
for (r = first ; r != end ; r = next)
{
if(sofartest())
{
nexttable.tsecer_select()
}
}
}

	

		

			
	

enum_nested_loop_state
sub_select(JOIN *join,JOIN_TAB *join_tab,bool end_of_records)
{
……
    error= (*join_tab->read_first_record)(join_tab);
    rc= evaluate_join_record(join, join_tab, error);
……
  while (rc == NESTED_LOOP_OK)
  {
    error= info->read_record(info);
    rc= evaluate_join_record(join, join_tab, error);
  }
……
  return rc;
}
static enum_nested_loop_state
evaluate_join_record(JOIN *join, JOIN_TAB *join_tab,
                     int error)
{
……
  if (select_cond)
  {
    select_cond_result= test(select_cond->val_int());
 
    /* check for errors evaluating the condition */
    if (join->thd->is_error())
      return NESTED_LOOP_ERROR;
  }
……
    if (found)
    {
      enum enum_nested_loop_state rc;
      /* A match from join_tab is found for the current partial join. */
      rc= (*join_tab->next_select)(join, join_tab+1, 0);
      if (rc != NESTED_LOOP_OK && rc != NESTED_LOOP_NO_MORE_ROWS)
        return rc;
      if (join->return_tab < join_tab)
        return NESTED_LOOP_OK;
      /*
        Test if this was a SELECT DISTINCT query on a table that
        was not in the field list;  In this case we can abort if
        we found a row, as no new rows can be added to the result.
      */
      if (not_used_in_distinct && found_records != join->found_records)
        return NESTED_LOOP_NO_MORE_ROWS;
    }
……
}

	

		

			
	

enum_nested_loop_state
sub_select_cache(JOIN *join,JOIN_TAB *join_tab,bool end_of_records)
{
  enum_nested_loop_state rc;
 
  if (end_of_records)
  {
    rc= flush_cached_records(join,join_tab,FALSE);
    if (rc == NESTED_LOOP_OK || rc == NESTED_LOOP_NO_MORE_ROWS)
      rc= sub_select(join,join_tab,end_of_records);
    return rc;
  }
  if (join->thd->killed) // If aborted by user
  {
    join->thd->send_kill_message();
    return NESTED_LOOP_KILLED;                   /* purecov: inspected */
  }
  if (join_tab->use_quick != 2 || test_if_quick_select(join_tab) <= 0)
  {
    if (!store_record_in_cache(&join_tab->cache))
      return NESTED_LOOP_OK;                     // There is more room in cache
    return flush_cached_records(join,join_tab,FALSE);
  }
  rc= flush_cached_records(join, join_tab, TRUE);
  if (rc == NESTED_LOOP_OK || rc == NESTED_LOOP_NO_MORE_ROWS)
    rc= sub_select(join, join_tab, end_of_records);
  return rc;
}

	

		

			
	

    if (!store_record_in_cache(&join_tab->cache))
      return NESTED_LOOP_OK;                     // There is more room in cache
    return flush_cached_records(join,join_tab,FALSE);

	

		

			
	

  - Store used fields from t1, t2 in cache
  - If cache is full

	

		

			
	

    - Read through all rows in t3
      - Compare t3 row against all t1, t2 combinations in cache
        - If row satisfies join condition, send it to client
    - Empty cache

	

		

			
	

static enum_nested_loop_state
flush_cached_records(JOIN *join,JOIN_TAB *join_tab,bool skip_last)
{
……
  info= &join_tab->read_record;
  do
  {//遍历t3表格所有记录
……
        for (i=(join_tab->cache.records- (skip_last ? 1 : 0)) ; i-- > 0 ;)
        {//遍历cache中所有t1、t2记录组合
          read_cached_record(join_tab);
          skip_record= FALSE;
          if (select && select->skip_record(join->thd, &skip_record))
          {//
            reset_cache_write(&join_tab->cache);
            return NESTED_LOOP_ERROR;
          }
          if (!skip_record)
          {//满足下推的where条件
//执行下一个table的遍历
            rc= (join_tab->next_select)(join,join_tab+1,0);
            if (rc != NESTED_LOOP_OK && rc != NESTED_LOOP_NO_MORE_ROWS)
            {
              reset_cache_write(&join_tab->cache);
              return rc;
            }
          }
……
  } while (!(error=info->read_record(info)));

	

		

			
	

mysql> create table harry (x int);
Query OK, 0 rows affected (0.03 sec)
 
mysql> insert harry values (1),(2),(3),(4),(5);
Query OK, 5 rows affected (0.00 sec)
Records: 5  Duplicates: 0  Warnings: 0
 
mysql> create table tsecer (y int);                   
Query OK, 0 rows affected (0.01 sec)
 
mysql> insert tsecer values (1),(2),(3),(4),(5);     
Query OK, 5 rows affected (0.00 sec)
Records: 5  Duplicates: 0  Warnings: 0
 
mysql> explain select * from harry, tsecer where x * x + y * y = 5 * 5;
+----+-------------+--------+------+---------------+------+---------+------+------+--------------------------------+
| id | select_type | table  | type | possible_keys | key  | key_len | ref  | rows | Extra                          |
+----+-------------+--------+------+---------------+------+---------+------+------+--------------------------------+
|  1 | SIMPLE      | harry  | ALL  | NULL          | NULL | NULL    | NULL |    5 |                                |
|  1 | SIMPLE      | tsecer | ALL  | NULL          | NULL | NULL    | NULL |    5 | Using where; Using join buffer |
+----+-------------+--------+------+---------------+------+---------+------+------+--------------------------------+
2 rows in set (0.00 sec)
 
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